The present invention relates to apparatus and methods for dispensing underfill material used during the attachment of semiconductor chips directly to printed circuit (xe2x80x9cPCxe2x80x9d) boards for flip chip on board or other substrates to form a chip package.
In the manufacture of PC boards it is frequently necessary to apply small amounts of viscous materials, i.e., those with a viscosity greater than fifty centipoise. Such materials include, by way of example and not by limitation, general purpose adhesives, solder paste, solder flux, solder mask, grease, oil, encapsulants, potting compounds, epoxies, die attach pastes, silicones, RTV and cyanoacrylates. Common methods of application have included screening, pin transfer and dispensing from a syringe or valve. Screening requires a template and is not readily adaptable to changing application patterns. Pin transfer is relatively fast but the tooling is expensive and inflexible and can only form dots, not lines of material. Many manufacturers use syringe dispensing with pneumatic mechanisms, electromechanical mechanisms or positive displacement valves.
Flip chip technology has developed as a result of the movement toward ever increasing miniaturization of circuitry. This technology is also known as direct chip attach or xe2x80x9cDCAxe2x80x9d. It includes xe2x80x9cflip chipxe2x80x9d bonding, dies attached directly to substrates, wire bonding, coated dies and encapsulated dies. One such process which is widely used is known as controlled columnar collapsed connection (xe2x80x9cC4xe2x80x9d).
Generally referring to FIGS. 1-3, a semiconductor die or flip chip 10 is provided with a pattern of solder bumps or balls 12 on an underside or circuit side thereof. The solder balls 12 are registered or aligned with solder pads 14 on a PC board or similar substrate 16. The underside of the chip 10 is also referred to as the image side of the chip. Flux (not shown) is normally supplied between the solder balls 12 and solder pads 14. Upon heating, the solder pads 14 on the PC board or substrate 16 reflow and physically connect with the solder balls 12 on the underside of the chip 10. The solder balls 12 typically have a high melting point and therefore do not reflow. This connection is illustrated diagrammatically in FIG. 2 by deformed solder pad 14xe2x80x2 mating with a solder ball 12. This process eliminates the requirement for wire bonding.
Since the flip chip 10 is not necessarily encapsulated in a plastic or ceramic package, the connections between the PC board 16 and the chip 10 can corrode. In order to prevent this corrosion, a special liquid epoxy 18 (FIG. 3) is used to completely fill the underside of the chip. This is referred to herein as the xe2x80x9cunderfillxe2x80x9d operation. Upon curing, the resulting encapsulation forms a non-hygroscopic barrier to prevent moisture from contacting and thus corroding the electrical interconnects between the PC board 16 and the chip 10. The epoxy 18 also serves to protect the bonds between the deformed solder pads 14xe2x80x2 and the solder balls 12 by providing thermal stress relief, i.e., accommodating different rates of thermal expansion and contraction. Stated another way, the cured epoxy 18 has a co-efficient of thermal expansion (xe2x80x9cCTExe2x80x9d) which, together with its bonding properties, minimizes the thermal stress induced by the difference between the CTE of the silicon chip 10 and the CTE of the PC board 16.
Advantages of using flip chips on board architecture include: 1) the potential for increased input and output (xe2x80x9cI/Oxe2x80x9d) as the entire die area beneath the chip is available for connection; 2) an increase in electronic processing speed due to shorter transmission line lengths; 3) the ability to fit a heat sink to the top of the chip; 4) a substantial reduction in chip profile; and 5) more efficient use of PC board real estate.
Referring to FIG. 3 of the drawings, once the underfill operation is complete, it is desirable that enough liquid epoxy be deposited along the edges of the chip 10 to fully encapsulate all of the electrical interconnections and so that a fillet 18a is formed along the side edges of the chip 10. Normally, the liquid epoxy flows under the chip 10 as a result of capillary action due to the small gap between the underside of the chip 10 and the upper surface of the PC board or substrate 16. If the chip is relatively large or the gap is unusually small, however, some of the electrical interconnections will not be encapsulated and voids 20 may exist. If such voids are present, then corrosion and undesirable thermal stresses may result.
It would therefore be desirable to provide a manner of underfilling the space between a semiconductor flip chip and a substrate, and especially in applications involving smaller than normal gaps or larger than normal flip chips, while preventing any voids or spaces left unfilled between the flip chip and substrate.
Generally, the invention relates to a method of underfilling a space between a semiconductor flip chip and a substrate to encapsulate a plurality of electrical connections with a viscous underfill material, such as epoxy resin. The method involves mounting the flip chip to the substrate with a plurality of electrical connections thereby forming a gap between opposed surfaces of the flip chip and substrate. The flip chip and substrate may be previously supplied in this form as an alternative. The viscous underfill material is dispensed adjacent at least one edge of the flip chip. The flip chip and the substrate are then rotated to move the underfill material into the gap under the influence of centrifugal force. This movement can additionally occur under the influence of capillary action or other means as well. In other words, the use of centrifugal force may only assist with the movement of material into the gap. The underfill material is then cured after fully encapsulating the electrical connections. In one manner of carrying out the invention, the substrate and flip chip may be mounted on a rotatable table or support such that the dispensed underfill material is located initially at an innermost edge of the flip chip with respect to the axis of rotation of the table. Centrifugal force will then act on the underfill material as the table rotates to move the material radially outward beneath the flip chip to entirely encapsulate the electrical connections.
As separate alternative aspects, which may or may not be combined, the flip chip and substrate may be heated at least during the rotating step to assist with the flow of underfill material and the flip chip and substrate may be rotated within a vacuum atmosphere to further reduce the occurrence of voids.
As another alternative manner of carrying out the invention, a dam is provided around more than one edge of the flip chip and opens toward the inner edge that receives the underfill material. In one embodiment, the dam may be formed from a gasket material, such as an elastomer, and may be temporary. That is, the dam may be removed after the centrifugal underfilling procedure. In another embodiment, the dam may be formed from curable material, such as an epoxy. This dam is permanent and is cured prior to rotating the flip chip and substrate. In each embodiment, the dam acts to contain the underfill material beneath the flip chip as the flip chip and substrate are rotated to encapsulate the electrical connections. For example, when the flip chip is a four-sided chip, the temporary or permanent dam may be provided adjacent three of the four edges, while the underfill material is dispensed adjacent a fourth edge disposed closest to the axis of rotation. This aspect of the invention is also applicable to other multi-sided flip chips having various shapes.
From the foregoing summary and the detailed description to follow, it will be understood that the invention provides a unique and effective manner of underfilling flip chips. The invention is particularly advantageous in flip chip applications in which very small gaps are formed between the flip chip and the substrate or in applications utilizing relatively large flip chips having a large space to underfill. In these situations, the capillary action normally relied upon to move the underfill material into the gap may not be enough to fully encapsulate the electrical connections and the centrifugal force utilized in the present invention can be used to ensure full encapsulation.
Additional features, advantages and objectives of the invention will become more readily apparent to those of ordinary skill upon review of the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.